Compared to a silicone solar cell which has been already put into practical use (monocrystalline silicone solar cell, polycrystalline silicone solar cell, or the like), a dye-sensitized solar cell (hereinbelow, referred to as DSC) has both high efficiency and low production cost, and thus it is highly recognized as a solar cell of third generation. Regarding the DSC, one of the important factors for improving power conversion efficiency is development and selection of a dye material. As a dye material which recently receives attention, a polypyridine Ru complex, which is a Ru (ruthenium)-based complex, is known (for example, the polypyridine Ru complex dye of Patent Document 1 or the like is known).
The step of manufacturing DSC includes coating paste containing TiO2 particles with nano size on a transparent conductive substrate followed by calcination to form a TiO2 layer, impregnating the substrate in a dye solution to adsorb dye on pores of the TiO2 layer, and forming a counter electrode and an electrolyte layer.
However, cost relating to dye, which is the main constitution of DSC, accounts for large portion of the cost for manufacturing DSC. In this regard, a Ru complex capable of exhibiting desirable performances as dye of DSC is often expensive. Since many of the Ru complex dye have a complex structure, it requires many steps from synthesis of a ligand to synthesis of complex dye. As such, when a final purification step is also included, the production cost naturally increases.
From the viewpoint of reducing the cost for manufacturing DSC, the present inventors tried recovering complex dye from a used dye solution which is discharged from a step of manufacturing DSC. Conventionally, a used dye solution discharged from a step of manufacturing DSC is generally discarded as a waste, and reuse has never been tried. Meanwhile, according to the study by the present inventors, it was confirmed that the use ratio of a dye solution (use amount of complex dye) is low for a step of manufacturing DSC and only several % of the complex dye in a solution is adsorbed onto a substrate per impregnation. Namely, according to a conventional step of manufacturing DSC, the used solution containing most of the complex dye is simply discarded as a waste liquid. In addition, if the complex dye can be recovered from a used solution, it can be said that the cost for providing the complex dye is reduced.